1. Field of the Invention
The present invention is directed to an inkjet printing system and method having an ink level detection system that uses changes in optical characteristics of a capillary ink containment material.
2. Description of the Related Art
Inkjet printers operate by ejecting small droplets of ink from a printhead onto a print medium. The printhead is mounted in an inkjet pen, which is held in the printer at the appropriate position with respect to the media. The ink must be presented to the printhead at the appropriate "backpressure," (i.e., less than ambient atmospheric pressure) to keep the ink from drooling from the printhead when the pen is not operating. Various mechanisms have been employed to contain the ink at the appropriate backpressure, including resilient elastomeric bladders, porous foam, internal accumulators, bubble generators, and spring-biased flexible bags. When the ink is depleted from the pen, it may advantageous or necessary to automatically sense the empty condition of the pen. For example, the system may be designed to automatically refill the pen by means of a flexible trailing tube or an automated refill station. In addition, damage may be done to the printhead if it is operated when the pen is empty.
Various mechanisms have been devised to sense the level of ink in inkjet pens. U.S. Pat. No. 5,751,300 (Cowger et al.), assigned to the present assignee, discloses an ink level sensor used in a trailing tube printer. A pair of electrical leads are implanted in a body of foam, and the current between the leads indicates ink level. The level of ink is used to operate a valve that controls the amount of ink allowed into the pen. U.S. Pat. No. 5,079,570 (Mohr et at.), assigned to the present assignee, discloses a binary fluidic indicator in a disposable print cartridge that uses a small tube or other element formed on the ink tank of an inkjet print cartridge. The main ink tank of the print cartridge is filled with a porous material such as polyurethane foam. This patent also mentions as alternative ink containing members: glass beads, felt pen fibers, capillary tubes, and rolled up plastic film (column 4, lines 15-18). The small element that provides the optical ink level indication does not contain the capillary material. When the ink level drops to a certain level, the capillary material draws the ink from the indicator, to thus provide a binary indication that the ink has dropped to a selected level. The indicator can be either human or machine readable. U.S. Pat. No. 5,406,315 (Allen et al.), assigned to the present assignee, discloses an optical sensor that detects the temperature and ink level based on changes in the reflectivity of a phase change material adjacent to or within the pen body housing.
Despite the above-mentioned and other ink level detection mechanisms, there remains a need for an inexpensive and reliable system for automatically detecting the level of ink in inkjet pens.